1. Field of the Invention
The invention relates to analytical methods and equipment for carrying out materials analysis. In particular, technology for performing dynamic mechanical analysis on non self-supporting samples is disclosed.
2. Background Information
Dynamic mechanical analysis (DMA) is an analysis technique for materials, particularly polymeric materials, over their linear viscoelastic regions. In DMA, an oscillating stress is applied to the material and the resulting strain measured. Two analysis modes are common. In one approach, a constant-frequency stress is applied to the material while temperature is being ramped. The resulting strain response over the temperature range may be interpreted to identify, e.g., higher-order phase transitions. In the other approach, the frequency of the oscillating stress is scanned while temperature at the sample is held constant. The strain response as a function of frequency may be interpreted to elucidate structural features and mechanical behavior of the material in different mechanical environments.
Traditionally materials have been investigated by DMA in a bulk form such as a bar or a sheet. However, the technique has been adapted for analysis of non self-supporting samples, which cannot be gripped directly by the DMA apparatus due to their size or consistency. Examples of non self-supporting materials include divided (e.g., powder or flaked) or nonrigid (e.g., thin film, semi-solid, or liquid) materials, by supporting the sample between metal plates, on the order of 0.002″ thick, to which the stress is applied during analysis. In a conventional configuration, known as a pocket, the plates are joined along a common edge.
However, DMA investigations of samples supported between metal plates may yield complex signals with experimental artifacts not seen in analogous scans of a corresponding bulk sample. The case of lactose is instructive. Lactose is commonly used as a carrier for inhaled drugs. Its glass transition temperature is of interest because lactose shelf life correlates positively with the glass transition temperature. A spray-dried form of amorphous lactose has been DMA-interrogated by loading particles into a pocket and cycling stress on the sandwiched sample while scanning temperature. In addition to an anticipated glass transition around 120° C., the storage modulus as a function of temperature shows a modulus decrease, consistent with a second transition, observed at a lower temperature. This suggestion of a second transition is an artifact of the experiment. There is, accordingly, a need to reduce experimental artifacts in dynamic mechanical analysis of non self-supporting samples.